Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical for formations containing small amounts of natural gas.
Formations that include small amounts of natural gas may include primarily oil, with the natural gas being a byproduct of oil production that is thus termed associated gas. In the past, associated gas has typically been flared, i.e., burned in the ambient air. However, current environmental concerns and regulations discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen.
Catalytic partial oxidation is one process used to form syngas and attempts to perform all of the partial oxidation reactions on a highly active catalyst in order to convert the hydrocarbon catalytically at a high rate. For example the contact times of catalytic partial oxidation may be on the order of milliseconds. Thus, for catalytic partial oxidation, it is preferable to premix a hydrocarbon-containing feed, such as methane or natural gas, with a molecular oxygen-containing feed at high temperature, pressure and velocity in order to enable the catalytic reaction to proceed at short contact times so that the chemistry occurs at the correct stoichiometry throughout the catalytic zone.
Therefore, an often desired component of a commercial scale operation is an apparatus to premix the hydrocarbon-containing gas, such as methane or natural gas, and the molecular oxygen-containing gas, such as air or substantially pure O2, at high temperature, pressure, and velocity. The same feed conditions that are conducive to efficient operation of the partial oxidation process, however, could lead to reactions that are less desirable, and possibly even hazardous, such as the ignition and combustion of the feedstock. At the same time, it is desirable to mix the feed gases as completely as possible, so as to maximize the efficiency of the catalytic reaction.
One problem with such mixing processes is that heated mixtures of oxygen and methane, at pressures of interest for syngas production, are highly reactive and can be explosive. Thus, it is often preferred to utilize techniques that increase the controllability of the process and to avoid pre-ignition and pre-reaction of the gases. One technique used in mixing the reactants is to place the mixing nozzles very close to the reaction zone such that there is a very short time between the reactants being mixed and contacting the catalyst. This technique often involves placing the mixing apparatus in close proximity to the reactor, which may make maintenance of the mixing apparatus difficult and requires that the mixer be designed to withstand the extreme environment of a partial oxidation reactor.
Another problem encountered in the design of these types of mixers is that high concentrations of oxygen, or oxygen rich gas, impacting components of the mixer at high velocities can cause damage to components of the mixer. This high speed contact can lead to oxygen impingement and a thermochemical reaction that may damage, and even destroy, components of the mixer.
Another concern in the design of mixing apparatus for catalytic partial oxidation reactions is allowing for the release of gases in the case of an emergency or backlight situation, where the gases in the mixing apparatus ignite and cause a dramatic increase in pressure. Because a catalytic partial oxidation reaction is potentially highly reactive, it is often preferred to provide for the release of gases in the event that the reaction becomes uncontrollable.
Thus, there remains a need in the art for methods and apparatus to improve the mixing of natural gas and oxygen to feed a catalytic partial oxidation process. Therefore, the embodiments of the present invention are directed to methods and apparatus for mixing that seek to overcome these and other limitations of the prior art.